Color correction modification has been in widespread use in connection with television advertisements and various aspects of film to tape transfer including preservation and restoration of color prints of deteriorating film media. Very sophisticated apparatus for finally selecting signal levels representing particular hues, and combinations of hue and saturation, in video signals have been developed. For example, color correcting apparatus shown in U.S. Pat. Nos. 4,642,682; 4,727,412; and 4,876,589 indicate that this is indeed a well developed art.
In recent years, much attention of members of television industries throughout the world have been turned to various proposals and apparatus for providing high definition television (HDTV) with both digital and analog composite signal methods.
As is well known to those skilled in the art, the NTSC standard broadcast television signal was adopted in the United States in 1941 and popularized shortly after World War II. In 1953, the current NTSC standard for color television broadcasting including a 3.58 megaHertz subcarrier carrying the chroma information, which composite signal was compatible with NTSC monochrome receivers, was adopted. Since continental Europe was recovering from the effects of World War II, it was somewhat later in adopting standard television signals. Most of Europe adopted a standard Phase Alteration by Line (PAL) composite broadcast signal with better resolution than that of the NTSC format. The French, being the French and needing to be different, adopted a color system known as SECAM.
Irrespective of the particular details of the standard for the composite signals, color television has labored in recent years under the burden of a composite signal specification which was state of the art per 1950 technology, but which has effectively retarded the development of higher quality television broadcasts for consumers.
Due to the ubiquitous spread, and tremendous cost reductions, of digital circuitry, the exchange of television and other video signals is most commonly accomplished by transmission of digitized video signals via satellite, microwave links and the like until such signals are converted to analog signals, and then composite broadcast television signals, for over-the-air transmission to home receivers.
In order to promote the international interchange of video signals and to standardize the interface between digital video signal sources and devices utilizing or transmitting same, the International Radio Consultant Committee (CCIR) promulgated Recommendation No. 601-1 in 1986 which defines a standard set of digitized color signals for television studios. CCIR Recommendation 601-1 (1986) is hereby incorporated by reference. The encoding of parameters as specified at sampling frequencies having the ratios 4:2:2. The fundamental frequency to which these ratios refer is derived from an analysis of common factors in the number of picture elements in NTSC 525 line and PAL 625 line television systems.
The standard signals are defined in Recommendation 601-1. Essentially, they consist of a luminance signal Y and two color difference signals (R-Y) and (B-Y). It is well known that, since the luminance signal contains information on levels of red, green and blue (R,G,B) that the three standard signals can be used to unambiguously reproduce the RGB levels for any given set of samples. As used in this specification, the term algebraic combination signal refers to any signal value for a color television system which is one signal of a set of signals which can be used to unambiguously derive RGB values. In other words, an algebraic combination signal is one of a set of signals which possess orthogonality such that unique and correct RGB values may be derived. Thus, the standard Recommendation 601-1 Y, (R-Y), and (B-Y) signals are a set of three algebraic combination signals. Similarly, R, G, and B signals form a set of algebraic combination signals.
It should be noted that appendices to Recommendation 601-1 contemplate systems in which the algebraic combination signals are sampled at ratios of 4:4:4. The 4:4;4 system contemplated by Recommendation 601-1 is one for which the signals are red, green and blue signals, rather than luminance and two color difference signals.
As noted hereinabove, color correction and modification is used in a number of applications within the television industry and other businesses which make use of video signals. Its principle use is in film to tape transfers and post production processing of commercials to highlight certain objects, colors and the like.
The discovery of the need of the present invention arose during work by the inventor with a digital color correction circuit. In digital color correction, the analysis to detect picture elements having particular hue and saturation characteristics to which correction or modification is to be applied is done by analyzing the values of various samples of digitized video signals. The principles involved are substantially the same as those used in analog domain color correctors.
The need to guard against aliasing in digital video signal processing is well known and recognized. CCIR Recommendation 601-1 includes a set of very specific requirements for anti-aliasing filters to be used with the digitization process in forming the standard sample sequences. So long as filters having the characteristics specified in the recommendation are used, the appearance of aliased frequencies will be kept to a minimum.
As is well known to those skilled in the art, the phenomenon of aliasing occurs in the digital processing of sampled signals. Aliasing is the name given to the phenomenon of the appearance of frequencies not present in the original signal that results from the presence in the sampled signal of frequency components which exceed one half of the Nyquist sampling frequency. Aliasing is particularly problematic in that it manifests itself as the appearance of non-existing frequencies in the resultant signal after conversion from the discrete time domain back to the continuous time analog domain. As is well known to those skilled in the art, the mathematics describing aliasing show that the spectrum of the aliased signals folds back on the spectrum of the signals of interest. Therefore, frequency components which lie slightly above one half the Nyquist value alias themselves as low frequency components. In digital video, distortion by the introduction of low frequencies manifests itself as shadows or the appearance of relatively large objects on the resultant picture. Thus, digitized visual images rapidly deteriorate in response to the appearance of aliased frequencies which can result from processing in the digital domain. This is the principle rationale for the rigorous specifications of anti-aliasing filters in Recommendation 601-1.
As noted hereinabove, the present inventor observed significant signal deteriorations in the resultant output when performing color correction in the digital, or discrete time, domain. It was discovered that significant changes in the saturation of colors possessing particular hues led to the appearance of low frequency distortion in the resultant output signal. Additionally, changes in gamma characteristics likewise led to distorted output results.
As is known to those skilled in the art, gamma (.gamma.) characteristics of a video system refers to the slope of a curve describing a transfer function. In television receivers, gamma is greater than one at most values of luminance. This non-linear characteristic is used to compensate for the non-linear response of the human eye. The gamma values at any point on the curve simply refers to the slope of the curve at that point. The problems of distortion which were observed by the inventor were not present in the use of analog color correctors. After studying same for some length of time, the source of problem was identified as harmonic distortion which results from the non-linear characteristics of many color correction and modification processes. For example, adjustment of the gamma characteristics of a video signal inherently provide a non-linear transfer function. As is well known to those skilled in the art, any non-linear system will produce harmonic distortion in the form of harmonics of frequency components of the input signal.
Additionally, the inventor realized that a number of color enhancement functions performed by color correctors are non-linear. For example, when it is desired to emphasize a particular object in a scene, wherein the object has a detectable hue that is distinguishable from the hues of other objects in the scene, the occurrence of picture elements containing this hue can be detected and the saturation level can be non-linearly increased. In conventional color correctors, this has the effect of "highlighting" the particular object. For example, the hue distinctive to a soft drink can in an advertisement can be saturated so as to draw additional attention of the viewer to the can.
Since color correction enhancement of this type is non-linear in that there is normally a stepwise increase in the saturation level in a portion of an image, this also produces harmonic distortion.
Also, it is known that any overflow of the value of digital samples in a digital signal processing device is analogous to the clipping of an analog signal. As is the case with the clipping of an analog signal, the clipping of the digital signal creates harmonic distortion. The inventor also believes that this is a source of the distortion of the video signals observed while working with digital color correctors.
Any of the mechanisms which generate harmonic distortion at frequencies which lie above one half the sampling frequency will lead to those frequency components appearing as aliased frequencies in the final converted analog signal. It is known to those skilled in the art that when one is designing a sampling system from the ground up, the technique of over sampling can be used in order to provide increased bandwidth to accommodate high frequency distortion components. So long as sufficient bandwidth is available in the digital system, any undesired high frequency components will maintain their proper spectral identity and can be filtered out in the final phases of digital-to-analog conversion. In other words, the signals will not appear at aliased frequencies if the sampling rate is sufficiently high. However, for CCIR 601-1 type sources of digital video signals, the sampling frequencies, particularly for those of the two color different signals having the lower sampling frequency, is close to the Nyquist rate. Therefore, there is very little spectral head room and color and gamma correction performed on video signals at these sampling rates will quickly lead to distortion in the form of aliased frequencies resulting from harmonic distortion when any significant non-linear change is made to the signal levels.
It is within the scope of the prior art to take CCIR 601-1 digitized signal streams, convert same to corresponding analog RGB signals, and then to perform the primary and secondary color corrections and modifications on the resultant analog signals. The corrected or modified signal can then be passed through a signal matrix and reconverted to a digital bit stream. However, this introduces additional quantization noise and causes the designer of color correction equipment to forego the potentially increased resolution available from operating in the digital domain. Therefore, there is a need for a signal processing system which can accept the near Nyquist rate sample sequences from a 4:2:2 CCIR 601-1 device, perform color corrections which include non-linear operations in the digital or discrete time domain, and then provide output signals in a format which meets Recommendation 601-1.